What do the children know most about Aslan?

I'm not sure what part of the story you're referring to here. From the first time they hear his name, the children have images of who and what Aslan might be. Edmund is filled with a sense of horror, whereas, Peter felt courage and suddenly desired adventure. Susun was felt the presence of beautiful music, and Lucy was reminded of holidays and summer vacation. Thus, they know his name alone provokes feeling they might not understand, but they eventually will.

You also have the fact that Mrs. Beaver advises the children that only the brave can fact Aslan without feelings of nervousness.